Accumulating anatomical, electrophysiological, and pharmacological data on the gustatory and visceral pathways suggest testable hypotheses about the functional organization of the brainstem taste nuclei. The PI's laboratory now has both in vivo and in vitro methods to investigate this organization. During the previous grant period, inhibitory interactions were shown in the neurophysiological responses of hamster parabrachial nucleus (PbN) cells to taste mixtures of sucrose and quinine or citric acid and both in vivo and in vitro studies demonstrated a GABAergic inhibitory network that acts on taste-responsive cells in the nucleus of the solitary tract (NST) of the hamster. GABA selectively inhibited sucrose responses to a greater degree than those to other stimuli. The effects of GABA appear to be mediated predominantly by the GABA-A receptor subtype. The techniques developed during the previous grant period provide means to address several fundamental questions about the neurotransmitters active in the taste pathway. Nothing is known about the transmitters between the primary gustatory afferent fibers and the second-order cells in the NST. Proposed studies will test the hypotheses that excitatory amino acids (EAAs) are involved in the synaptic processing of gustatory information from the peripheral nerve to the NST, that cells sensitive to different taste qualities use different transmitters, and that peptides (SP or CGRP) cooperatively interact with EAAs in this system, especially on the response to quinine. There are strong inhibitory interactions within the brainstem between sweet and bitter stimuli and between VIIth and IXth nerves. Studies will test the hypothesis that these interactions are mediated by the GABAergic circuit that the PI demonstrated within the gustatory NST in the previous funding period. Cell within the NST are subject to a number of descending modulatory influences, including a direct projection from the gustatory cortex (GC). Experiments will test the hypothesis that the descending modulatory influences from GC that enhance or inhibit NST taste responses are mediated via different transmitter systems. To investigate these proposals, the PI will combine neurophysiological recording from single taste-responsive cells in vivo with local microinjection of neurotransmitter agonists and antagonists. Afferent input from the peripheral nerves, interactions among different stimuli, and the descending regulation from GC will be studied by selective stimulation of these systems and manipulation of their pharmacology. NST cells will be identified as PbN projection neurons or non-projection neurons by antidromic activation from the PbN. Once evidence for a neurotransmitter effect is demonstrated in vivo, its mechanisms of action will be investigated by intracellular and patch-clamp methods in vitro.